At present full duplex digital data transmission between two subscribers connected to the same central office requires the use of two nonloaded wire pairs. That is, at present, when it is necessary or desirable for two subscribers to have the capability of simultaneously transmitting digital data between each other (commonly called full duplex digital data transmission capability), two pairs of nonloaded wires running between each subscriber and the central office are required. The two pairs of wires are jumpered at the main distribution frame of the central office so that one pair of wires can carry transmissions in one direction and the other pair of wires can carry transmissions in the other direction. At the subscriber's locations, the pairs of wires are connected to data terminals via modems (modulator-demodulator systems). Various data transmission techniques are utilized to transmit digital data over the two pairs of so connected wires, including relatively low Frequency Shift Keying (FSK) techniques.
One of the major disadvantages of the foregoing type of data transmission system is the need for two pairs of wires. More specifically, telephone operating companies (TELCO's) charges to subscribers are based in part on the number of pairs of wires running between the central office and each subscriber's premises. As a result, a data transmission system requiring two pairs of wires is substantially more expensive than a comparable data system requiring only a single pair of wires. It is an object of this invention to accomplish this result.
That is, it is an object of this invention to provide a digital data carrier system that provides for the simultaneous transmission of digital data signals in both directions over a single pair of wires.
In addition to the two wire pair requirement for full duplex digital data capability, at present, usually, a subscriber needs at least one additional pair of wires if conventional telephone communication is desired. That is, usually, the digital data wire pairs used to carry digital data, carry digital data only and an additional pair of wires is needed for voice communication, when simultaneous digital data and voice communication is desired. As a result, the subscribers costs are further increased. It is an also an object of this invention to overcome this disadvantage. That is, it is also an object of this invention to provide a communication system suitable for simultaneously transmitting digital data and voice signals over a single pair of wires.
It is a more specific object of this invention to provide an audio and full duplex digital data carrier system suitable for transmitting digital data between two subscribers connected to the same central office and audio communication between the subscribers and yet other subscribers all via single pairs of wires extending between the subscribers and the central office.
As will be better understood from the following description, the present invention contemplates transmitting digital data in Frequency Shift Keying (FSK) form. As will be readily understood by those familiar with FSK digital data transmission, such transmission requires the production of a signal that shifts between two different frequencies. In order to minimize the bandwidth used by this transmission method, it is necessary that the phase of the sine waves of the transmitted signals be maintained coherent when shifting from one frequency to the other. It is a further object of this invention to provide an FSK transmitter that accomplishes this result. That is, it is a further object of this invention to provide a new and improved FSK transmitter that maintains the coherence between the phases of generated sine waves when changing from one FSK frequency to another FSK frequency.
As will also be understood by those familiar with FSK data transmission, when full duplex transmission on a single pair of wires is to take place, in addition to being at different frequencies, the waveforms of the transmitted and received signals must be such that they do not include harmonics at the other signal frequencies. Or, if such harmonics are included in the waveforms, the level of such harmonics must be so low as to be undetectable.
Therefore, it is a still further object of this invention to provide a full duplex FSK digital data transmission system wherein the transmitted and received FSK signal pairs are at different frequencies and wherein the waveforms of the various FSK signals are very low in harmonics at or near the fundamental frequencies of the other FSK signals.
It is yet another object of this invention to provide a full duplex FSK digital data transmission system wherein the waveforms of a pair of transmitted FSK signals are low in harmonics at or near the frequencies of a higher pair of received FSK signals.
It is yet a further object of this invention to provide a new and improved FSK transmitter.
It is yet a still further object of this invention to provide an FSK transmitter that produces a pair of FSK signals at different frequencies, each of said FSK signals having a generally sinusoidal waveform that is low in harmonics at other predetermined frequencies, and that maintains phase coherence between the pair of FSK signals when shifting from one signal to the other signal.